For the restoration of tire treads on various vehicles, a variety of pre-cured treads have been designed for use in tire retreading processes. FIG. 1 illustrates an exemplary pre-cured tread as used on a retread tire 10. Tire 10 includes a tire carcass 12 with a crown section 14, reinforcement 16 and sidewall 18. Crown section 14 may have been buffed or otherwise worked to provide a prepared bonding surface to which a laterally extending tread 20 can be bonded. Buffing old tread off of the tire in preparation for retreading removes rubber that is discarded as waste. Much of this waste rubber is typically replaced during the retreading process as part of the undertread portion of the tread that is bonded to the carcass.
A tread 20 that is applied to carcass 12 includes respective top and bottom faces 20a, 20b and a predetermined thickness T coextensive therewith. Tread 20 also extends between opposing lateral sides 20c that may be coextensive with tire carcass 12. One or more tread elements 21 are integral with tread 20 in a variety of configurations as known in the art to impart predictable and repeatable performance characteristics to any tire upon which tread 20 is employed.
Tread 20 further incorporates a plurality of longitudinal grooves 22 that may be provided in communication with one or more lateral grooves (not shown) as known in the art. Grooves 22 have a predetermined width delineated by opposing sides 21a of adjacent tread elements 21. Each groove 22 terminates at a groove trough 22a that is offset by a distance D from tread bottom face 20b. A layer of bonding material 24 is arranged between bottom tread face 20b and tire carcass 12. Bonding material 24 may be selected from any amenable boding material, including but not limited to bonding rubber and adhesive.
In retreading operations, distance D often corresponds to an undertread 26 having a thickness at or about equal to distance D (i.e., the thickness of the undertread of pre-cured tread 20). It is understood that the undertread thickness of the pre-cured tread is differentiated from the total tire undertread thickness that is obtained after completion of a retreading process.
Undertread 26 of tire 10 is positioned vertically (i.e., perpendicular to the ground contacting surface of the tread) between groove troughs 22a and reinforcement 16. Undertread 26 provides a cushion and/or protective layer between the ground contacting tread and reinforcement 16 in tire carcass 12, thereby ensuring reinforcement 16 is not exposed through the wearing of the grooved tread area.
A skid depth S is generally defined by a difference between thickness T between top and bottom tread faces 20a, 20b and the thickness of the undertread of pre-cured tread 20 (e.g., distance D). Skid depth may therefore be identified and measured as the thickness of useful tread contained within the pre-cured tread (i.e., a thickness purposely rendered available for wearing during vehicle operations). Pre-cured treads often contain an undertread having a thickness from about 3.0 to about 4.0 mm inclusive to provide rigidity and stability during processing. Not only is this quantity over and above the actual skid depth: the undertread is added to the bonding layer and the material over the belts (e.g., reinforcement 16 as shown in FIG. 1) on the tire being retreaded. Thus, the total undertread of the retreaded tire becomes the sum of these three layers.
During retreading processes, tires are encased in flexible covers (or “envelopes”) and disposed in an autoclave. Air in the autoclave is pressurized (e.g., from between 1 to 6 bars) to force the new tread against the carcass and thereby achieve a uniform, tight bond. The air is heated to a temperature sufficient to produce the vulcanization reaction of the bonding layer interposed between the tread and carcass. While reducing the thickness of the undertread (e.g., reducing distance D) would realize desired weight savings in the pre-cured tread, thinly fabricated undertreads have heretofore led to decreased stability of the tread structure against the bonding layer during the curing operation because of the action of the envelope.
One solution is disclosed by co-owned U.S. Pat. No. 4,934,426 that is directed to curing of a pre-cured tread on a carcass without use of an envelope (the entire disclosure of U.S. Pat. No. 4,934,426 is incorporated by reference herein). A network of shallow channels provided along a bottom surface of the pre-cured tread facilitates removal of all air between the two surfaces upon application of a vacuum. Bonding is ensured initially by atmospheric pressure and subsequently by high pressure within an autoclave within which the assembly is treated. Such process beneficially eliminates air from between the tread and the carcass without addressing skid depth.
Additional benefits may therefore be realized by a pre-cured tread having a reduced undertread thickness that reduces weight and discarded rubber without sacrificing effective skid depth. A retread method that employs such a tread would optimize carcass sculpture in concert with tread sculpture to ensure a skid depth at least equal to that of prior treads of greater weight and thickness.